A new medical syndrome, Acquired Immune Deficiency Syndrome (AIDS) has recently been recognized. It is characterized by profound progressive depression of the immune system, resulting in repeated opportunistic infections and at least one type of neoplasm. The syndrome is usually if not always fatal. It is epidemic in the United States among certain "high-risk populations (male homosexuals, illicit drug users, Haitians, and, to a lesser extent, hemophiliacs, recipients of blood transfusions and intimate contacts of cases). A new retrovirus has been identified as the etiologic agent of AIDS. SUccessful transmission of the virus to chimpanzees has been reported. Attempts to find a more suitable primate animal model are in progress.